Warmth and healthy menu give city spiders big edge on country cousins

SPIDERS living in warmer urban environments are getting significantly bigger than their country cousins, who have less prey to chose from and spend more energy keeping warm.

SPIDERS living in warmer urban environments are getting significantly bigger than their country cousins, who have less prey to chose from and spend more energy keeping warm.

At the Ecological Society of Australia's annual conference in Melbourne on Wednesday, a University of Sydney PhD candidate, Lizzy Lowe, will outline her research, which suggests urbanisation has a positive impact on some species.

She studied the golden orb weaver in three types of environments in and around Sydney - urban parks, remnant bushland and continuous bushland. Twenty sites were studied and for each spider web found, she assessed its proximity to man-made objects and vegetation.

After comparing 222 spiders - including measuring size, weight, body fat - Miss Lowe found city spiders were bigger than those from the bush. She also established that within urban environments, spiders were bigger when they were found closer to man-made objects.

''It's probably to do with heat, so if they are near a car park, then it radiates heat and the warmer the environment, the quicker they develop,'' she said.

Miss Lowe said availability of prey would also likely be a factor, as insects are attracted to lighting in cities, parks and gardens. This makes a lamp post prime real estate for web-builders.

A common spider in urban environments in eastern states, the golden orb weaver builds a large, strong yellow silk web - sometimes up to 80 cm in diameter. Females can grow up to 10 cm, while males are smaller.

Miss Lowe said the urban heat island theory would likely apply to other spiders commonly found in urban areas such as the redback spider and daddy long-legs.

She said larger urban spiders were nothing to fear. ''Spiders eat flies, caterpillars and aphids, they are really good for urban environments,'' she said. ''If they're not in these environments then basically insects go crazy and over-eat vegetation and the whole ecosystem goes out of sync.''

The term urban heat island refers to increased warmth caused by the buildings, pavements and asphalt retaining heat. The temperature difference between urban and rural areas is often most noted at night when the heat continues to radiate long after the sun has set.

Plants may also grow differently in cities compared with country areas. Plant physiologist Stephanie Searle led a Columbia University project on tree growth in New York. The research - which looked at the heat island effect on New York's native red oak trees - found higher night-time temperatures may boost plant growth.

"The city is our baseline for what might happen in future decades, and with all the negative effects global warming may have, there may be a bit of a silver lining," she told The New York Times last week.